Fresh air flor modulation device

ABSTRACT

A device for monitoring and modulating the admission of fresh air into a building. The device comprises a housing; a sensor associated with the housing and operable to measure airflow through the housing; and a damper including at least two damper blades. Each damper blade includes a shaft rotatably attached to the housing at least at a first shaft end. The shaft is located on the surface of the damper blade so as to divide the surface into first and second substantially equal areas.

This application is a division of application No. 08/377,321, filed Jan.24, 1995, now U.S. Pat. No. 5,674,125.

BACKGROUND OF THE INVENTION

The present invention is directed to a flow modulating device used tocontrol and monitor airflow. In particular, the device is intended tomodulate and monitor the amount of fresh air being provided to the airconditioning system of a building. In an alternative embodiment, thedevice also mixes the fresh air with return air. Circuitry to improvethe resolution of airflow measurement is also described.

Indoor air quality (IAQ) affects everyone involved with building design,construction, operation, use and maintenance. Sick buildings can resultin sick occupants; sick occupants are less productive, don't renew theirleases, and tend to sue everyone involved.

Consequently, building system designers must comply with therequirements of ASHRAE Standard 62-89 "Ventilation for Acceptable IndoorAir Quality". In broad terms, ASHRAE 62-89 specifies requirementsintended to control microbial growth and to provide adequate ventilationfor contaminant dilution. Included are two methods of providing adequateventilation: the ventilation rate procedure and the indoor air quality(IAQ) procedure. The ventilation rate procedure specifies minimumoutdoor airflow rates for adequate dilution, while the indoor airquality procedure specifies contaminant levels and subjective evaluationfor acceptable indoor air quality. Most designers choose the ventilationrate procedure since it is prescriptive and seems less risky than theIAQ procedure. Designers tend to avoid the IAQ procedure because itseems open ended and subject to multiple interpretations.

The ventilation rate procedure provides a prescriptive path tocompliance. It defines acceptable indoor air quality in terms of minimumventilation airflow for contaminant dilution. A table lists ventilationairflow requirements for spaces and a simple equation determines systemlevel outdoor airflow requirements for multiple space systems. Manydesigners prefer the ventilation rate procedure since it presents anobjective ventilation design approach.

Devices for controlling the intake of fresh air are known. For example,U.S. Pat. No. 4,200,117 to Anderson et al. shows a device including apair of closure members pivoted relative to a transverse axis and biasedto the closed position by a spring. U.S. Pat. No. 5,324,229 toWeisbecker shows a two section damper including a fresh air inlet and amixing section to prevent stratification. U.S. Pat. No. 5,276,630 toBaldwin et al. also discloses an outside air connection controlled by adamper. The Baldwin et al. patent and the Weisbecker patent are assignedto the assignee of the present invention and are hereby incorporated byreference.

SUMMARY OF THE INVENTION

There is a need to supply fresh air in known volumes. There is also aneed to mix fresh air with return air. There is a need to close thefresh air damper in the event of a power loss. There is also a need tocontrol airflow in a linear manner as the fresh air damper opens. Thereis also a need to improve the measurement of fresh air airflow.

The present invention provides a device for monitoring and modulatingthe admission of fresh air into a building. The device comprises ahousing; a sensor associated with the housing and operable to measureairflow through the housing; and a damper including at least two damperblades. Each damper blade includes a shaft rotatably attached to thehousing at least at a first shaft end. The shaft is located on thesurface of the damper blade so as to divide the surface into first andsecond substantially equal areas.

The present invention further provides an airflow modulation devicecomprising a housing supporting a flow ring; a first damper blade; and asecond damper blade. Each of the first and second damper blades includesa first edge which is substantially straight, a second edge which issubstantially arced and a shaft being attached to each damper blade inthe plane of the damper blade. The shaft is movably attached to thehousing and located so as to divide the surface of the damper blade intofirst and second surfaces. The area of the first surface issubstantially identical to the area of the second surface.

The present invention also provides a flow swirl generator comprising ahousing and at least two arced damper blades. Each blade is centroidallysupported by a shaft where the shaft has a first end rotatably supportedby the housing and a second end rotatably supported by a central pivotpoint.

The present invention additionally provides a method of measurementcomprising the steps of: measuring a value; forwarding measured value asan analog signal; dithering the analog signal to improve resolution; andconverting the analog signal to a digital signal.

The present invention still further provides a method of providinganalog to digital a conversion comprising the steps of: receiving ananalog signal; dithering the analog signal with a known signal; takingmultiple samples of the dithered signal with an analog to digitalconverter; and interpolating a resultant digital signal.

The present invention yet further provides apparatus for providing 13bit resolution of a measured analog signal using an 8 bit analog todigital converter. The apparatus comprises an analog sensor whichprovides a measured signal on an output line A; a filter circuitreceiving the analog signal; a dither circuit superimposing a knownsignal on the filtered analog signal; and a microprocessor and analog todigital converter for receiving and sampling the dithered and filteredanalog signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an air conditioning system suitable for usewith the present invention.

FIG. 2 is a view of the flow modulation device of the present inventionfrom the interior of a building.

FIG. 3 is a perspective view of the flow modulation device of thepresent invention.

FIG. 4 is a side view of the flow modulation device of the presentinvention.

FIG. 5 shows an individual damper blade in accordance with the presentinvention.

FIG. 6 is a view of a gear train drive release in the powered positionas used with the present invention.

FIG. 7 is a view of the gear train release of FIG. 6 shown in theunpowered position.

FIG. 8 is a side view of the gear train release of FIG. 6.

FIG. 9 is a graph of damper position versus airflow for prior artdevices and for the present invention.

FIG. 10 is an alternative embodiment of the present invention taken inthe open position as viewed from the interior of a building.

FIG. 11 is the alternative embodiment of FIG. 10 with the dampers in theclosed position.

FIG. 12 is the alternative embodiment of FIG. 10 showing a single blade.

FIG. 13 is the alternative embodiment of FIG. 10 showing the axis andcentral connection.

FIG. 14 is an end on view of the single blade of FIG. 12 viewed from theaxis of the shaft.

FIG. 15 is a graph of analog signal in accordance with an aspect of thepresent invention.

FIG. 16 is a circuit diagram in accordance with the invention asdescribed with respect to FIG. 15.

FIG. 17 is a table of the signal described with reference to FIG. 15.

FIG. 18 is a flow chart of the invention described with respect to FIG.15.

FIG. 19 is an exemplary signal superimposed on an analog signal inaccordance with the invention described with respect to FIG. 15.

FIG. 20 is a graph of the compensation table of the invention withreference to FIG. 15.

FIG. 21 shows a mixing module adapted for use with the alternativeembodiment of FIGS. 10 through 14.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to a flow modulation device forcontrolling and measuring the amount of outdoor air provided to an HVACsystem. In an alternative embodiment a flow swirl generator is alsodescribed.

FIG. 1 shows a typical HVAC system 10 including two zones 12 and 14whose air is to be conditioned. Typically, the air is conditioned, i.e.heated or cooled, by an air conditioning unit 16 which conditions air bymeans of a coil 18. Such air conditioning units for both heating andcooling purposes are conventional and well known to a person of ordinaryskill in the art. Refrigerant is transferred by means of conduits 20between the coil 18 and the air conditioning unit 16. The conditionedair, i.e. supply air, is impelled by an air handling device such as afan 22 through ducting 24 to the zones 12, 14 whose air is beingconditioned. After use in the zones 12, 14, the return air is returnedby ducting 26 and either exhausted from the building by an exhaust 28 orrecirculated by means of recirculation ducting 30. An exhaust damper 32is provided to control the amount of air being exhausted by the exhaust28 and a recirculating damper 34 is provided to control the amount ofair being recirculated through the recirculation duct 30. Additionally,the recirculated air is mixed with outdoor air received from an outdoorair intake 36. The outdoor air passes through and is controlled by anoutdoor air damper 38 and is then mixed with the recirculated air. Themixture is directed by supply air ducting 40 to the coil 18 where theair is conditioned and the air conditioning cycle continued.

The present invention is directed to a flow modulation device 50 asshown in FIGS. 2, 3, 4 and 5. The flow modulation device 50 essentiallyreplaces the outdoor air damper 38. Outside air enters from 46 andleaves the flow modulation device 50 at 48. The flow modulation device50 includes a housing 52, preferably circular, supporting a flow ringsensor 54 such as is shown in U.S. Pat. 4,344,330 to Renken et al., thispatent being assigned to the assignee of the present invention andhereby incorporated by reference. The flow ring sensor 54 is provided toaccurately measure the amount of outside air being introduced, althoughother flow measuring arrangements are also contemplated. A flange 55 maybe provided to attach the device 50 to an outside wall or, to provide asmoother transition in entering airflow, a bell mouth 57 may beprovided.

The flow modulation device 50 also includes a damper section 51 having apair of damper blades 56. Each damper blade 56 is mounted on its ownshaft 58, and the shaft 58 rotatably supported by the housing 52. Alinking mechanism 60 ensures that the damper blades 56 move in mirrorimage synchronization. A drive mechanism 62 drives one of the shafts 58and the linkage mechanism 60 causes the other damper blade 56 to followthe movement caused in the first damper blade 56. A critical aspect ofthe present invention is the centroidal location of the shaft 58 withrespect to the surface 70 of the individual damper blade 56. As shown inFIG. 5, the shaft 58 is located on the surface 70 of the damper blade 56such that the area 71, 72 of the surface 70 on either side of the shaft58 is substantially the same. In other words, the area 71 issubstantially equal to the area 72 for each damper blade 56.Effectively, the damper blade 56 is easy to modulate since air pressureis balanced on either side of the shaft, and therefore provides finecontrol with minimal control force.

In the preferred embodiment, the shape of each damper blade 56 isgenerally half circular with a peripheral edge 73. The peripheral edge73 includes a substantially straight edge 76 and a curved edge 78. Agasket 74 preferably is attached to the peripheral edge 73 of eachdamper blade 56 so as to provide a seal when the damper blades 56 are inthe closed position as shown in FIG. 2. For ease of assembly andinsulative effect, the gasket 74 is preferably in two pieces, each of asize and shape to be sandwiched between the areas 71 on either side ofthe damper blade 56, or the areas 72 on either side of the damper blade56. Preferably a sealing support 75 supported by the housing 52 andaligned with the straight edges 76 is provided to act as a blade stopwhen the damper blades 56 are in a closed position. Alternative sealingarrangements include mirror image lips formed in the damper blades 56along the straight edge 76, or other conventional sealing engagementssuch as the gasketless air damper shown in U.S. Pat. No. 4,917,350 toBeyer. This gasketless air damper is assigned to the assignee of thepresent invention and is hereby incorporated by reference.

As noted and unlike other previous butterfly dampers, the shaft 58 isnot located at the straight edge 76, nor is the shaft 58 located in themiddle of the damper blade at the midpoint 80 of a perpendicular line 81from the midpoint 82 of the straight edge 76 as one might expect.Rather, centroidally locating the shaft 58 to form the balanced areas71, 72 ensures that the damper blades 56 turn easily in response to thevarious pressures passing through the duct 40.

The centroidal location of the shaft 56 with respect to the surface 70of the damper blade 56 has an additional unexpected advantage in thatairflow is linear across the range from the damper's closed position tothe damper's open position. This is illustrated in FIG. 9 with respectto both the present invention and the prior art as shown by graph 90.Airflow is shown on the X axis 92 while damper position is shown on theY axis 94. On the Y axis 94, the location 96 illustrates the fullyclosed damper position while the location 98 illustrates the fully opendamper position. Similarly, on the X axis 92, location 96 alsoillustrates the zero airflow position while location 100 illustrates themaximum airflow position. In the prior art, as can be seen from the line102, airflow is non-linear with respect to damper position. A smallchange in damper position from the closed position 96 to a position 104results in a significant nonlinear increase in airflow as shown byposition 106. More specifically, the prior art devices let a lot of airthrough when they are initially opened.

However, airflow in the present invention is linear as shown by line110. The relation of airflow to damper position is linear, where a smallchange in damper position such as from location 96 to location 104results in a small and linear change in airflow as indicated by point112. Small amounts of airflow can be closely controlled with linearairflow control. The present invention's linear relationship across theentire damper is illustrated by the line 110.

FIGS. 6, 7 and 8 show, in more detail, the drive mechanism 62 which alsoincludes a motor drive gear train release 128 to ensure that the damperblades 56 are placed in a closed position in the event of a power loss.A motor 130 is mounted on a pivoting plate 132, pivoting around a screw134. The screw 134 is attached to a base 136 which supports a solenoid138. As shown in FIG. 7, when in the unpowered mode, a spring 140 pushesthe plate 132 in a canted position such that the motor drive gear 142 isphysically separated from the driven gear 144 by a gap 146.

As shown in FIG. 6, when power is provided to the solenoid 138, thespring 140 is compressed as the solenoid pulls the plate 132 toward thesolenoid and engages the motor drive gear 142 with the driven gear 144.As illustrated in FIG. 8, the motor 130 is preferably on the oppositeside of the base 136 from the gears 142 and 144.

An alternative embodiment of the present invention is shown in FIGS.10-14 as a flow swirl generator 150. This alternative embodimentperforms the monitoring and modulation of outdoor air as does the flowmodulation device 50, but the flow swirl generator 150 of thealternative embodiment also induces a swirl into the outdoor air so asto facilitate turbulent mixing with recirculated air from the duct 30.

FIG. 10 shows the flow swirl generator 150 in a partially openedposition and FIG. 11 shows the flow swirl generator 150 in a fullyclosed position. In this alternative embodiment, four damper blades 152are shown although a minimum of two blades is contemplated.

Each blade 152 is centroidally mounted on a shaft 154 so that the area156, 158 on either side of the shaft 154 is substantially equal, as waspreviously described in connection with the primary embodiment. Eachblade 152 is formed as a curved surface 159 as is shown best by FIG. 14where the blade 152 is viewed along the axis of the shaft 154. For thefour blade flow swirl generator 150, the curved surface 159 of eachblade 152 has a smooth arc 151 greater than 90° and preferably 100° orgreater. Each blade 152 is formed with a substantially straight leadingedge 160, a substantially straight trailing edge 162, and a curved edge161. The straight edges 160, 162 meet at the shaft 154 and divergetherefrom at an angle greater than 90° for a four blade flow swirlgenerator 150. The curved edge 161 joins the diverged ends of the edges160, 162, the curved edge 161 having an arc adapted to the shape of thehousing 52.

When in the closed position shown in FIG. 11, the trailing edge 162 andthe leading edge 160 of adjacent blades 152 have an area of overlap 153which provides a better seal. On the curved surface 159, the leadingedge 160 of the arc faces into the direction of the entering outdoor airand the trailing edge 162 of the arc is pointed at an angle ranging from90° to 180° relative to the leading edge 160. In this regard, the angleis selected to be closer to 90° when more turbulence is desired, andselected closer to 180° when less turbulence is desired. This curvedsurface 159 of the damper blade 152 creates a swirling effect in theentering outside air which causes it to turbulently mix withrecirculated air for better mixing effect.

In a three blade flow swirl generator, each blade has a smooth arcgreater than 120° between the leading edge 160 and the trailing edge162. The arc of the curved surface 159 of the damper blade 162 isselected closer to 120° to create more turbulence and selected closer to240° to create less turbulence.

As shown in FIG. 13, the individual shafts 154 are not as long as thediameter of the housing 170 of the flow swirl generator 150. Instead,the length of each shaft 154 is approximately that of the radius of thehousing 170. This is because opposing damper blades 152a and 152b willturn in opposite directions so that each blade 152 induces swirl in thesame direction. A first end 172 of each shaft 154 is rotatably mountedin the housing 170 and a second end 174 is rotatably engaged with acentral support 176. The central support 176 is located approximately atthe center point of the housing 170 and is held in position by rods 177affixed between the central support 176 and the housing 170. An actuatorring 180 is used to control the movement of the damper blades 152,although other controlling mechanisms are also contemplated. Theactuator ring 180 is movably linked to a connecting rod 182 which inturn is attached to the first end 172 of the shaft 154, preferablyoutside the housing 52. The connecting rod 182 is rigidly attached tothe first end 172 by welding or by other conventional fasteners such asbolts. A conventional linear actuator 184 provides rotational force tothe actuating ring 180 as an arm 186 of the actuator 184 is moved in adirection tangent to the actuator ring 180. This causes the shaft 154 torotate with the movement of the connecting rod 182 and thereby open orclose the flow swirl in response to the linear actuator 184. Theactuator arm 186 is movably attached to the actuator ring 180 using anyconventional device or fastener 188. Additionally, other ways andapparatus for controlling the movement of the damper blades 152 arecontemplated including individual actuators or motors on the first ends172 or an actuator mounted in association with or on the central support176.

The flow swirl generator 150 may be applied in a mixing module 400 whichfacilitates easy field installation. The mixing module is shown in FIG.21 and includes a housing 402 in a shape suitable for easy installation,e.g. rectangular or cubic. The housing 402 includes a fresh air inputaperture 404 in which the flow swirl generator 150 is mounted, arecirculated air input aperture 406 adapted for connection to therecirculation ducting 30, and a supply air egress aperture 408 adaptedfor connection to the supply air ducting 40. Arrow 410 indicates thedirection of fresh airflow, arrow 412 indicates the direction ofrecirculating airflow and arrow 414 indicates the direction of supplyairflow.

The housing 402 forms a chamber 416 where the recirculating air and thefresh air are thoroughly mixed due to the turbulating effect of theblades 152 of the flow swirl generator 150. The thoroughly mixed airexits through the supply air egress aperture 408 into the supply airducting 40. If additional mixing effect is desired, the housing 402 maybe extended by adding an extended mixing housing 418 as shown by dashedlines 420. With the removal of a wall 422 containing the supply airegress aperture 408 and the addition of a similar supply air egressaperture 424 in the extended mixing housing 418, an extended mixingchamber 426 is added to the mixing chamber 416. In general, the anglebetween the selected edge 160 and the trailing edge 162 is selectedcloser to 90° to generate more turbulence in the smaller volume ofchamber 416, and the angle is selected closer to 180° in the greatervolume of the combined extended mixing chamber 426 and mixing chamber415. Basically, more turbulence is preferred in smaller volumes as toensure thorough mixing. This is true of three blade, four blade, fiveblade and other blade systems built in accordance with the teachings ofthe present invention.

Another aspect of the present invention relates to improving themeasurement of a value such as airflow through the flow modulationdevice 50 by improving the resolution of the analog to digitalconversion of the electronic signal corresponding to the signalgenerated by the flow ring sensor 54. This aspect also applies to themeasurement of other values such as temperature, humidity, voltage,current, or the like. FIG. 15 shows a graph 200 having a first level ofdiscrimination 202 and a second level of discrimination 204. Any signalreceived between the first and second levels of discrimination 202, 204cannot be distinguished due to the limitations in signal discrimination.For example, an analog signal represented by the line 206 as sampled atpoint 208 is indistinguishable from an analog signal represented by theline 210 as sampled at point 212.

However, a known signal can be superimposed upon the analog signal usingcircuits such as that shown in FIG. 16. Circuits or circuitry aredefined herein to include hardware, software, and/or firmware.Superimposing such a known signal on the analog signals 206 and 210respectively results in modulated signals 214 and 216. Whereas bothsignals had previously been in the area 220 between the lines 202 and204, portions of the modulating signal 214 now falls below the line 202in the area 222 and portions of the modulating signal 216 now fall abovethe line 204 in the region 224. The portion of time that the signal 214below discrimination line 202 is represented by the region 226, whilethe portion of time that the line 216 above the line 204 is representedby the region 228. By averaging the signal portions respectively in theregions 220, 222 and 224, the position of the analog signal 212 or 208between the lines 202 and 204 can be interpolated. Two analog signals210, 206 are shown merely to provide separate examples demonstrating howthe signal represented by the line 206 can be interpolated as closer tothe lower discrimination level line 202 while the signal represented bythe line 210 can be interpolated to be closer to the discriminator line204. In practice a single analog signal 210 or 206 is received and thetime in each region 222, 224, 226 determined and used as a basis forinterpolation. Alternatively and particularly more difficult, the actualareas can be integrated. In practice, it is much easier to time thesignal 210 or 206.

This is further illustrated with respect to FIG. 17 using the Table 230.The first column A of the table illustrates the analog signal lines 206and 210. In the second column B, the discrimination between lines 202and 204 shows that each of these signals 206, 210 will fall in area 220and be effectively indistinguishable from each other. Using the presentinvention is demonstrated in column C where the portion 226 of themodulated signal 24 is found to lie X % in area 222 and the remainder tolie Y % in area 220. This allows an interpolation as to the location ofthe analog signal 206 with respect to lines 202 and 204 to bedetermined. Similarly, the modulated signal 216 can be determined to beZ percent in area 220 and W percent in area 224 allowing aninterpolation of where the analog signal 210 is with respect to thelines 202 and 204.

The circuit 250 to accomplish this is shown in FIG. 16 and is relativelysimple. The analog signal 252 enters on line 254 and is filtered by aresistor 256 and a capacitor 258. The filtered result is forwarded tothe analog to digital converter of a microprocessor 260 by means of anelectrical connection 262. An output line 264 of the microprocessor 260having a resistor 266 and an optional capacitor 268 modulates the analogsignal on line 262. Preferably, the signal pulsed on line 264 is a fiftypercent duty cycle signal which results in a a triangle shaped or sawtoothed shaped signal on line 262 due to the long constants of theresistors 266, 256 and the capacitor 258. The use of the optionalcapacitor 268 eliminates imposing a small DC offset on the originalsignal. In the preferred embodiment, the resistor 256 is a ten K ohmresistor while the resistor 266 is a 1.5 megaohm resistor. The capacitor258 is a 4.7 microfarad capacitor while the capacitor 268 is a 0.22microfarad capacitor.

The signal from the microprocessor 260 on line 264 is a fifty percentduty cycle squarewave with a 200 millisecond period. This signalmodulates the signal on line 262 producing a signal such as thatrepresented by the line 308 in FIG. 19. The signal on line 262 issampled as shown by the flow chart 300 in FIG. 18. The flow chart beginsat 302 and initializes a sum at 304 to be a value of zero. 128 eight bitinput samples are taken and summed at step 306 over the next 200milliseconds.

The sum sampled at 306 was found to give an interpolation error whichvaried according to the strength of the analog signal being input at254. It is compensated for at step 310 by adding an error correctionfactor directly related to the magnitude of the modulated signal 308.This correction factor is empirically determined and entered in a lookuptable where the modulo 128 of the sum from step 306 is used as the entryvalue into the lookup table. FIG. 20 is a graph of the compensationfactor versus modulo 128 of the sum. The compensation factor is appliedat 310 and the flow chart 300 exited at 312.

The resultant analog to digital technique provides better than 13 bitresolution from an 8 bit analog to digital converter. The compensationstep 310 alone provides more than an eight times improvement. The flowchart 300, with the compensation step 310 omitted, provides a resolutionby itself of approximately ten bits from an eight bit sample. Thethirteen bit resolution makes possible accurate airflow measurementsfrom the flow ring 54 ranging from under 100 feet per minute(approximately 0.001 inches water column) to 2500 feet per minute(approximately 1 inch water column). Thus the flow modulation device ofthe present invention can measure down to five percent of unit airflow,as opposed to the minimum accurate readings of 15 percent unit airflowwithout the entire technique shown in FIG. 18.

The present invention as directed to a flow modulation device and itsalternative embodiment of a flow swirl generator have been disclosed interms of the specific embodiments described above. A person of ordinaryskill in the art will recognize that many alterations and modificationsare within that persons skill. Such modifications specifically includevarying the number of damper blades for both disclosed embodiments tooperate with two, three, four or more damper blades. Other modificationsinclude modifying the shape of the housing 52 to elliptical,rectangular, square or other shapes. Additionally, circuitry as usedherein is defined to include software, hardware and firmware,implemented either independently or in combination. A person of ordinaryskill in the art will recognize that all such modifications andalterations are contemplated to fall within the spirit and scope of thefollowing claims.

What is claimed for Letters Patent of the United States is asfollows:
 1. A device for monitoring and modulating the admission offresh air into a building, the device comprising:a housing; a sensor,associated with the housing and operable to measure airflow through thehousing; and a damper including at least two damper blades wherein eachdamper blade is substantially planar with a first substantially straightedge and a second curved edge; wherein each damper blade includes ashaft rotatably attached to the housing at least at a first shaft end;wherein the shaft is located on a surface of the damper blade so as todivide the surface into first and second substantially equal areas; andwherein the shaft is not located on the surface at the midpoint of aperpendicular line taken from the midpoint of the first straight edge.2. The device of claim 1 further including a linking mechanism causingsaid at least two damper blades to move in synchronized but mirror imagefashion.
 3. The device of claim 2 wherein the first substantiallystraight edges of said at least two damper blades engage.
 4. The deviceof claim 3 wherein the housing is circular and the second curved edgesof the at least two damper blades sealingly engage the housing.
 5. Anairflow modulation device comprising a housing, the housing supporting:aflow ring; a first damper blade; a second damper blade; wherein each ofthe first and second damper blades includes a first edge which issubstantially straight, and a second edge which is substantially arced,a shaft being attached to each damper blade in the plane of the damperblade, the shaft being movably attached to the housing and located so asto divide the surface of the damper blade into first and second surfaceswhere the area of the first surface is substantially identical to thearea of the second surface; and wherein the shaft is not located on thedamper blade surface at the midpoint of a perpendicular line taken fromthe midpoint of the straight edge.
 6. A flow modulation devicecomprising:a housing; at least a pair of damper blades each damper blademoveably supported within the housing by a shaft rotationally associatedwith the housing at a first shaft end; and a flow ring sensor mountedwithin the housing and operable to measure airflow through the housing;wherein each damper blade has a semicircular shape which is separated bythe shaft into two regions of approximately equal area each damper bladehaving a straight edge and a curved edge, the shaft being locatedparallel to the straight edge but not at the midpoint of a perpendicularline taken from the midpoint of the straight edge.
 7. The device ofclaim 6 wherein the housing is substantially circular and each shaft isrotationally associated with the housing at a second shaft end.
 8. Thedevice of claim 7 further including an actuator operable to controlmovement of the damper blades, the actuator being located external ofand associated with the housing.
 9. The flow modulation device of claim6 wherein the damper blades have a shape which is generally halfcircular with a peripheral edge where the peripheral edge includes astraight portion and a curved portion.
 10. The flow modulation device ofclaim 9, the flow modulation device having an open position and a closedposition, wherein airflow is linear across the range from the openposition to the closed position.
 11. The flow modulation device of claim10 wherein a small change in damper position results in a small andlinear change in airflow.
 12. The device of claim 1 wherein the shape ofeach damper blade is half circular.
 13. The device of claim 12 wherein asmall change in damper blade position results in a small and linearchange in airflow through the device.
 14. The device of claim 13 whereineach damper blade has a range of positions and airflow is balanced oneither side of the shaft of each damper blade for each damper bladeposition.